Electron microscope specimens are, as a rule, phase objects which generate only a very slight amplitude contrast in a transmission electron microscope because of the high electron energy in the range of 100 keV and higher. For this reason, in a conventional transmission electron microscope, objects of this kind generate a contrast only when utilizing the phase-shifting effect of the spherical aberration of the transmission electron microscope and are therefore imaged with correspondingly little contrast in a conventional transmission electron microscope. The introduction of a phase plate into the back focal plane of the objective of the transmission electron microscope therefore provides a large increase in contrast in a manner similar to the generation of a phase contrast in phase objects according to Zernicke in the optical microscopy. However, the dimensions required in a transmission electron microscope are problematic. Especially when the so-called unscattered ray (that is, only the ray which is undiffracted at the specimen) is to be shifted in phase, but the ray, which is diffracted at the specimen into the first order or higher orders, is intended to be uninfluenced by the phase plate, the small diameter of the unscattered ray of less than 1 μm imposes considerable requirements on the technology because, for the phase plate, freedom from contamination, freedom of charging and dielectric strength are required.
For generating the phase-shifting effect, basically two starting points are known, namely: for the first starting point, the phase plate is realized as a correspondingly small configured electrostatic lens which imparts a phase shift only to the unscattered ray but leaves the higher diffracting orders entirely or substantially uninfluenced. For the second starting point, a thin foil is used which is substantially transparent for electrons of the electron energy used in the transmission electron microscope and which has the required structure. For the second starting point, the inherent electrostatic potentials of the material are used in order to impart the desired phase shift onto the unscattered ray or onto the scattered electron rays. The first starting point has the disadvantage that the small electrostatic lens perforce requires outer holding structures which interrupt regions wherein the paths of the electrons run which are diffracted into high diffraction orders whereby important information is lost for the generation of images. With respect to the latter, the second starting point has the disadvantage that the higher orders of diffraction, which are anyway very weak compared to the unscattered ray, are additionally weakened by the unavoidable material absorption of the foil. Because of these technological problems, no phase contrast electron microscopes with phase-shifting elements directly in the back focal plane of the objective lens could, up to now, be successfully established in the marketplace as commercial products even though the basics for the generation of phase contrast have been known for more than fifty years.
Phase contrast electron microscopes are described in U.S. Pat. Nos. 6,744,048 and 6,797,956 which are incorporated herein by reference.
In U.S. Pat. No. 6,744,048, the suggestion is made to image the back focal plane of the objective by a lens system and to arrange the phase-shifting element in the image plane of the diffraction plane of the objective with the image plane being generated by the lens system.